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I have a project where I have a main board with a possible mezzanine board connected to it through a connector.

The main board has a 12V supply and got a DC/DC for 3.3V voltage (call it 3.3VMAIN) to power components.
The mezzanine board is also provided with 12V supply and got it's own DC/DC for 3.3V voltage (call it 3.3VMEZ).
GND is the same for both boards, connected through connector and attachment screws.

Main board and mezzanine are communicating through some GPIOs and a SPI bus (running 1-5MHz max).

I was wondering:

  • If I could connect SPI devices of the main board directly to SPI devices of the mezzanine

enter image description here

  • OR if I need to add some open collector buffers due to the fact I don't have the same power supply reference (3.3VMAIN vs 3.3VMEZ) enter image description here

One of my concern is that I could get some reverse voltage from one the 3.3V rail to the other one in case of the board is not powered on. Second concern is to have open collector buffer with pull-up on all SPI bus lines.

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  • \$\begingroup\$ Don't describe your circuit, draw it. \$\endgroup\$
    – Andy aka
    Feb 19 at 16:18
  • \$\begingroup\$ @Andyaka I edited the post with schematics of my two proposals. GND is common to both board and not represented. \$\endgroup\$
    – zeqL
    Feb 19 at 16:56

1 Answer 1

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SPI, 5MHz through a mezzanine, no problem. If you have the pins, could alternate with grounds just to keep cross-talk minimized. Although not needed for short distances, treating the SPI traces as length-matched transmission lines wouldn't hurt for longer runs.

As for voltage differences, if both 3.3V are always within about 0.2V, the level should be fine. Check all datasheets for absolute max values; if the max is 0.3V higher or lower than any other pin, then 0.2V gives some margin of safety.

However, be wary of one supply coming up first, especially switching types which can take a variable amount of time to stabilize. If one goes high before the other, and other I/O pins are involved, then this could cause problems back-feeding the "down" chip through I/O pins, up to and including latch-up and destruction. This also means the boards are not hot-swappable, unless such a mezzanine exists to make stable power connections first.

A power supervisor chip might help with power-on synchronizing. But if the 3.3V Main regulator can be oversized, and 3.3V sent through the mezzanine, that will be safer and simpler.

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  • \$\begingroup\$ Thanks. The fact is the power supply of 3.3V main is in fact on another board and I'm limited in current. So I need some additional DC/DC pour the mezzanine board. I think I'll put some buffer with open-drain output in case there are some issue with power supplies. Like I said, I'm worrying about reverse voltage. \$\endgroup\$
    – zeqL
    Feb 20 at 16:16

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